The present application is related to and claims priority from UK patent applications GB0913015.4 (SURESENSORS) and GB0916067.2 (SURESENSORS) filed on 27 Jul. 2009 and 12 Sep. 2009 respectively.
Diabetes is one of the most widespread non-infectious diseases. It is estimated that around 246 million people suffer from diabetes and that each year another 7 million people develop the disease. The complications associated with diabetes include an increased risk of suffering a heart attack, stroke, blood circulation disorders, kidney damage, blindness and nerve conduction disorders.
Assessing the concentration of glucose in the blood is an established and effective way of managing diabetes. Diabetics, in particular insulin-dependent diabetics, are advised to monitor their blood glucose levels several times a day in order to adapt and improve treatment plans. Due to the number of times blood glucose levels should be measured, it is highly preferable that diabetics are able to self monitor blood glucose levels without the need for medical supervision.
Home-use assay systems such as those for the monitoring of blood glucose have made significant advances in recent years to reduce the sample volume and assay time. However, it is still possible to get significantly erroneous results due to a wide range of reasons. These reasons include incorrect strip storage, environmental factors, interfering factors in the sample and/or unusually high or low haematocrit levels. The problem is exaggerated if more than one of these factors happens to be present at the time of testing. One solution that manufactures are pursuing is to try to measure in a separate test one or more of the factors that can affect strip response and then correct for any extreme in the measured factor. This method relies on accurate measurement of the factors inducing error and a universally applicable algorithm for error correction.
US2009/0177406 (BAYER; HUAN-PING discloses a biosensor system capable of adjusting a correlation for determining analyte concentration from output signals form one or more index functions extracted from the output signals.
Disposable electrochemical glucose sensors have been available for many years and are described in numerous patents including U.S. Pat. No. 5,288,636 (POLLMANN et al), WO01/073124 (INVERNESS MEDICAL; DAVIES et al.) and U.S. Pat. No. 7,276,147 (ROCHE; WILSEY), U.S. Pat. No. 6,284,125 (USF FILTRATION; HODGES et Al.); WO97/18465 (MEMTEC; HODGES et al.), U.S. Pat. No. 6,241,862 (INVERNESS MEDICAL; McALEER et al.) and U.S. Pat. No. 6,193,873 (LIFESCAN; OHARA et al.). These systems typically use a disposable test sensor, for example in the form a strip that is inserted into a meter. Once a measurement has been carried out the test strip is removed and thrown away. In other words this is a single use disposable sensor. However the currently available systems are all prone to give erroneous results when used near the extremes of more than one of their stated operating ranges (for example with a sample at the low end of the haematocrit range and with an unusually high level of an interfering substance) or when misused in some way. It is known that some users store strips outside of the original packaging resulting in inaccurate readings. One means of mitigating these occurrences and therefore creating a more reliable measurement is the use of an internal calibration that looks for a known reading from an internal standard included in each test strip. This approach has been described in WO 2005/080970 (PA CONSULTING; NOBLE), WO2008/029110 (SURESENSORS; DAVIES) and US2007/0287191 and WO2006/015615 (both from EGOMEDICAL; STEINE et al) for example.
Sensor designs sometimes use water-insoluble membranes to retain reagents at the electrode surface or to provide a barrier to potential interferents (e.g. WO93/15651 ELI LILLY; ALLEN). WO93/15651 (ELI LILLY; ALLEN) discloses “acrylic copolymer membranes for biosensors” where “the membranes of the invention show good adhesion to substrates in an aqueous environment and possess excellent wet-strength.”
US2003/0178322 (AGAMATRIX; IYENGAR et al.) discloses the use of a variable potential waveform that is applied to the test strip and signal analysis used to try to determine the effects of interfering factors as opposed to the glucose response. US2009/0184004 (LIFESCAN; CHATALIER et al.) discloses the use of resistance as an indicator of haematocrit and the use of an algorithm to try to correct for the effects of haematocrit.
WO97/38126 (MERCURY DIAGNOSIS; DOUGLAS et al.) discloses a glucose test strip containing “ . . . a water insoluble polymeric layer capable of blocking the passage of red blood cells and allowing the passage of blood fluids containing an analyte . . . .”
An alternative use for membranes in diagnostic devices is the use of porous membranes to increase the surface area available to support immobilised or adsorbed reagent. For example WO02/08763 (USF FILTRATION; HODGES et al.) discloses the use of macroporous membranes for immunosensors and also discloses that “The protein or antibody may be contained within a matrix, e.g. polyvinyl acetate. By varying the solubility characteristics of the matrix in the sample, controlled release of the protein or antibody into the sample may be achieved. The support structures described are insoluble or very poorly soluble in water.
The use of a blood cell or interferent exclusion membrane as typically used in biosensors requires that they remain intact in the presence of an aqueous sample.
Electrochemical glucose strips are typically constructed by coating one or more of the detection surfaces with the reagent. In addition to the active ingredients of enzyme and mediator the reagent formulation typically also contains non-reactive ingredients that confer properties required for the manufacturing method or confer desirable properties to the test strip. Typically sensors are made by depositing the reagents onto the detection surface as a liquid and subsequently drying the reagent layer. Deposition of the liquid reagent can be done by a variety of methods such as screen-printing, single drop liquid dosing or ink-jet printing.
US2003/116447 (SURRIDGE et al) discloses the use of an interdigitated array disposed on a flexible substrate and states that “A preferred method for applying the chemistry matrix to the sensor chamber (IDA) is a discrete dispense of 500 nanoliters of the coating solution into the 1 millimeter×4 millimeter chamber . . . ”.
U.S. Pat. No. 5,288,636 (POLLMAN et al.) also discloses “6 μl of reagent made by the above protocol is added to well 9 formed by cutout 8. This amount of reagent 11 will substantially cover surface areas 10 on both electrodes . . . .”
The use of disintegratable films for diagnostic devices has been disclosed in WO2005/040228 (ADHESIVES RESEARCH; MEATHREL et al.) in which it is stated that “A disintegratable film containing one or more reagents can improve the stability of the reagents. Additionally, the reagents can be used more effectively and efficiently, since the film can be localized to a particular area within the testing device and can be handled easily as compared to an aqueous solution. Further, providing reagents in film form promotes efficient use and minimises reagent wastage since film can be divided into individual segments having a desired amount of reagent and the need for spraying, coating, or striping a reagent can thus be eliminated, if desired.”
WO2005/080970 (PA CONSULTING; NOBLE) discloses a concept with a specific sequence of the detection areas in the flow path. “In a preferred arrangement, the detector means includes at least two detectors, a first of said detectors being arranged to detect the analyte level in the unadulterated sample, and a second of said detectors being arranged downstream of said predetermined amount of the analyte to detect the analyte level in the calibration sample. In one embodiment of the above arrangement, said first and said second detectors are arranged in series on the flow path, and said predetermined amount of the analyte is located between said first and second detectors. Thus there may be a single flow path with the fluid passing, in order, the first detector, the predetermined amount of the analyte and the second detector.” It goes on to add “it is also preferable that the calibration glucose on the sensor strip mixes quickly and homogeneously with the blood sample which passes it.”
WO2008/029110 (SURESENSORS; DAVIES) discloses the use of multiple internal standards but does not disclose a practical means of designing an electrochemical test strip
WO 2006/015615 (EGOMEDICAL; STIENE et al.) discloses a diagnostic device that contains multiple internal standards each within separate sample channels. The predetermined amount of analyte used as the standard is positioned on the opposite face of the sample chamber to the active reagents. This achieves appropriate separation of internal standard analyte from the reagent(s) however creates substantially different transport paths for the analyte from the sample that reacts with the reagent(s) (which will react very close to the working electrode providing very short diffusion paths) as opposed to that from the internal standard which will not react until it meets reagent(s) perhaps somewhere in the bulk sample.
US2006/0024835 (LIFESCAN; MATZINGER) discloses a photometric glucose measurement system that uses reagents spread onto an insoluble support matrix. These insoluble matrices slow down diffusion of reagents and lead to slow assay times of 45 seconds. Such assay times are now commercially unacceptable when compared to the current industry norm of about 5 seconds. The present invention is designed in one exemplary embodiment to achieve rapid test times of less than 10 seconds in an electrochemical assay format.
WO2005/080970 (PA CONSULTING: NOBLE), WO2008/029110 (SURESENSORS; DAVIES), WO2006/015615 (EGOMEDICAL; STIENE et al.) and US2006/0024835 (LIFESCAN; MATZINGER) disclose aspects of the internal standard idea applied to diagnostic test strips. However, these all provide only partial solutions or solutions that have some important disadvantages in the implementation of the internal standard method compared to the present invention.
The art disclosed above does not address issues of provision of an internal standard addressed by one or more embodiments of the present invention. The analyte, glucose in one example, is already dissolved in the test sample and so the reactive ingredients of the test strip are solubilised into the sample already containing the analyte. The inventor has appreciated that the situation with the internal standard test is different in that there is a quantity of analyte in the sample plus an additional ‘standard’ level of the same analyte that must dissolve into the sample before being measured. Preferably this extra step has no effect on the measurement efficiency of the standard.
Typically sensor production methods deposit a wet reagent formulation onto a detection area by means of wet film casting, liquid dosing or screen printing techniques. If this wet reagent formulation contacts the standard then there is the likelihood that some initial reaction occurs with the internal standard analyte while the reagent layer is drying. This is undesirable as it creates variation that would be detrimental to the use of an internal standard.
A method of manufacturing a sensor having a reagent and internal calibration standard therein is required that reduces and perhaps eliminates to any appreciable extent the risk of any unintended reaction taking place during sensor production and/or later during sensor storage prior to use.
One or more aspects of the invention seek to provide a solution to the problem of unintended reaction between the reagent and the standard dose of calibration analyte, especially when the analyte of interest is the same as the calibration analyte.
One or more aspects of the invention seek a solution to the problem of separating the internal standard and the reagent without introducing differences in their respective transport paths to a measurement electrode.
Further, one or more aspects of the invention seeks a solution to the problem of timing of the reaction between the analyte of interest and the reagent, and timing of the reaction between the reagent and the calibration analyte Such problems may include starting at different times or being of different duration, and perhaps adversely affecting the measurement of the analyte of interest.
The invention also seeks to provide a sensor having small sample volumes and short test times.